In the operation of earth-moving vehicles, such as front-end loaders, a common work cycle is that of loading the bucket from a pile and unloading it into a truck. In such a work cycle, the operator of the loader approaches the pile in second gear from a travel portion of the work cycle and initially crowds the pile with the bucket in second gear. Then the operator usually shifts to first gear as the load on the engine is increased by prying or bucket-lifting operations. When the bucket is loaded, the operator reverses the transmission and backs the loader away from the pile, starting in first gear and then upshifting to second gear for the travel portion of the work cycle. In the travel portion, the front-end loader is commonly turned around to approach the truck in the forward direction, the turn-around being executed by decelerating in reverse on a turning radius and then accelerating in forward on a diverging turning radius. In the turn-around maneuver, downshifting of the transmission from second to first gear for acceleration in the forward direction may impose a severe shock on the driveline of the vehicle with resulting damage. It is therefore desirable to complete the reversal while in second gear and then downshift to first gear for acceleration in the forward direction. The transmission may be upshifted to second during the travel to the truck for unloading of the bucket. For returning to the pile, the loader is reversed and performs the turn-around maneuver as described above.
Such a work cycle, as just described, requires repeated shifting between first and second gears and repeated direction reversals. The manual shifting is very demanding on the operator and results in inefficient operation. Accordingly, front-end loaders are commonly provided with an automatic shift control system so that the transmission is shifted between first and second gears in response to a vehicle speed signal with an upshift at a predetermined speed value and a downshift at a lower speed value. Such a transmission shift control system is disclosed in U.S. Pat. No. 4,301,689 which is assigned to the same assignee as this invention.
While the automatic shift control system as described above is effective to relieve the operator of repeated shifting between first and second gears, a work cycle which requires a deceleration-reversal-acceleration pattern may result in downshifting during reversal and damage to the driveline. It is therefore desirable to provide additional automatic control which will prevent downshifting during reversal but will permit upshifting during reversal since it is not harmful to the driveline.
Automatic means for reducing the shock on the driveline during reversal have been proposed in the prior art.
In the Peppel U.S. Pat. No. 4,164,876, assigned to the same assignee as this invention, a downshift inhibitor circuit is provided for a powershift transmission. As disclosed in the Peppel patent, the purpose of the downshift inhibitor is to prevent downshifting when a reversal is executed at full throttle. When a reversal is executed at part throttle a downshift does occur within a predetermined time after a downshift signal is generated. This system comprises a delay circuit connected between a speed sensor switch and the solenoid shift valves. The direction control lever actuates a reversing switch on each reversal. When the speed sensor switch calls for second gear, actuation of the reversing switch starts a timer which prevents actuation of the solenoid shift valve until a predetermined time delay after the speed sensor switch shifts to first gear. Accordingly, when the reversal is made at full throttle in second gear, the vehicle will accelerate fast enough during the predetermined time interval so that the speed is back up to the second gear speed range at the end of the time delay and no downshift occurs. On the other hand, if the reversal is made at part throttle, the vehicle speed will still be in the first gear speed range at the end of the time interval and the downshift will occur.
It is a general object of this invention to provide a downshift inhibitor which overcomes certain disadvantages of the prior art.